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Geometry and Di usion. Comment on \Tracer Di usion in a Dislocated LamellarSystem"1, 2, yDoru Constantin and Robert Ho lyst1Laboratoire de Physique de l’ENS de Lyon, 46 Allee d’Italie, 69364 Lyon Cedex 07, France2Institute of Physical Chemistry PAS, Dept. III, Kasprzaka 44/52, 01-224 Warsaw, PolandPACS numbers: 61.30.Jf, 66.30.JtIn a recent Letter, Gurarie and Lobkovsky [1] a rm shown that :that the movement of tracer particles across the layers2pin a lamellar system with screw dislocations is super- D = D (2)? 2 2(2r) + pdi usiv e (ballistic when dislocations of one sign are in ex-2cess and with normal displacement hz (t)i / t log t whenClearly, the particle always exhibits normal di usionthe average dislocation charge is zero). We argue that,along z, with a di usion constant D D, even when?when surface geometry is properly taken into account,r ! 0 (incidentally, in this limit one obtains the \pipethe normal displacement is di usiv e and the associateddi usion" along the dislocation core). On the contrary, ifdi usion constant D is always smaller than the in-plane? 2one neglects the p term in the denominator of equationt D.(2) (as in reference [1]), the ratio D =D arti cially di-?Particle di usion on a curved surface extending indef-verges with vanishing r. This example is very relevant toinitely in all three directions in space is characterized by2 2 2 the discussion in [1] because, as the authors acknowledge,a displacement hx +y +z i = D t, where ...